1. Field of the Invention:
This invention relates to taximeters.
2. Description of Prior Art:
Mechanical taximeters generally employ two rotatable elements, one of which is driven by a motor at a constant rotational speed, and the other of which is driven usually indirectly, from the taxi wheels or gear box or speedometer cable at a rate which is proportional to the speed of the vehicle. Clutch means is provided operating to couple an output shaft to whichever of the two rotatable elements is rotating at the greater speed. This shaft drives a metering and display mechanism to display a fare charge corresponding to the number of rotations of the output shaft. The indicated fare will thus advance at a constant rate when the vehicle is stationary as the output shaft will be driven at a constant speed or "time" rate by the rotatable element which is coupled to a motor. On the other hand, when the vehicle is travelling at a speed greater than a predetermined speed, the fare is advanced at a rate proportional to the distance or "mileage" being travelled by the vehicle, the output shaft in this case being driven by that rotatable element which is coupled to the taxi wheels, gear box or speedometer cable.
Mechanical taximeters are somewhat disadvantageous in that they employ numerous parts which are prone to wear and require constant servicing. This difficulty has been sought to be overcome by providing electronic digital taximeters which replace the mechanical clutch, the rotatable elements and the motor for driving one of the elements by a fixed frequency oscillator, to represent the time rate, and a variable frequency generator, generating a signal the frequency of which is varied in proportion to the speed of the vehicle. These oscillators produce pulse streams, and suitable electronic selector means is provided to select whichever of the two pulse streams is at the highest frequency and apply this to advance a fare couter. Whilst these arrangements require virtually no moving parts, and are inherently quite reliable, they are still disadvantageous in that it is possible to lose a pulse from each pulse stream during transitions caused by switching of the selected pulse stream from the mileage pulse stream to the time pulse stream. Such lost pulses result in loss of revenue and the loss can be significant in cases where frequent mileage time transitions occur, such as in city driving. An object of the invention is to at least substantially avoid this difficulty.